The present invention relates generally to apparatus and methods for absorbing or suppressing vibration in rotating machinery, and more specifically to a centrifugal pendulum absorber for attenuating resonant vibration in jet engine blades over all engine speeds.
Potentially destructive resonant vibration can occur when the frequency of an alternating excitation force imposed on a machine part from the motion of the machine equals a natural, or resonant, frequency of the machine part. A typical machine part will have more than one resonant frequency, representing the various modes, or ways, in which the machine part can vibrate. For example, a jet engine blade attached to the circumference of a jet engine rotor may be viewed as a simple cantilever beam which, as it rotates, experiences various bending forces, such as the aerodynamic forces, or kick, imposed on it each time it rotates past a stator (a stationary blade). As a cantilever beam, it will have resonant frequencies representing various bending modes in which it will simply bend, or whip, back and forth, bend with one wave along its length, bend with two waves along its length, and so forth, with each higher resonant frequency typically being an integer multiple of a first resonant frequency. If excited by an alternating force having an excitation frequency the same as a resonant frequency of the blade, the excitation force will cause increasingly greater blade oscillations in that vibration mode, similar to pushing a playground swing at the natural frequency of the swing. The frequency of the bending forces, or the excitation frequency, imparted on jet engine blades is typically, and logically, an integer multiple of the rotation rate of the jet engine. For example, one usual source of an excitation force is, as previously described, the aerodynamic kick imposed each time a blade passes a stator. This will happen a certain set number of times with each complete rotation. Another excitation force may be from the aerodynamic kick imposed from the interaction with other rotor blades and stators upstream from the blade of interest. This force will also happen a set number of times per rotation, but likely a different set number of times from the first described source. Each of these physical sources of a vibratory excitation will cause a different so-called speed line, which is an integer multiple of engine speed, or rotation rate. As first described by W. Campbell in a pioneering 1924 work describing the problems of vibration in turbomachinery, this can be described in a Campbell diagram, where speed lines are plotted on a graph against the various frequencies at which resonant vibration will occur for each vibration mode of the machine part in question. Wherever a speed line, corresponding to a particular aspect of turbomachine structure which causes a regularly occurring vibratory excitation, crosses a resonant frequency line, resonant vibration is possible. This means that the excitation frequency on jet engine blades from operation of a jet engine will equal or coincide with, in a stepwise manner, successively higher resonant frequencies of the blades as engine speed increases.
Modern jet engine blades are better modeled as plates than as beams, so that they have more complicated vibration modes, including, in addition to conventional bending modes, torsion modes and chordwise bending modes. All these vibration modes combine to determine the actual resonant frequencies for a particular jet engine or other turbine or turbomachine blade.
The prior art for reducing resonant vibration in jet engine blades has primarily concentrated on damping a particular vibration mode or family of modes. This is usually accomplished by adding friction dampers. Unfortunately, friction damping is difficult to characterize and can deteriorate with wear. Other damping research for engine blades has focused on using viscoelastic materials. The major challenges with viscoelastic treatments are the limited availability of materials that can function in an elevated temperature region and the tendency of those materials to creep when subjected to the centrifugal force field created by a rotating turbine. The most successful viscoelastic approach for rotating components has been the application of vitreous enamel coatings with internal three-dimensional metallic containment networks.
The jet engine prior art has also studied vibration absorbers and particle dampers. It has been shown, for example, that a damped cantilevered beam located on the tip of a blade would effectively suppress vibration for a particular vibration mode.
The non-jet engine prior art has also explored the use of centrifugal pendulum absorbers, which are pendulums attached to rotating components (such as a crankshaft for an automotive-type internal combustion engine) to absorb various bending and torsional vibration modes of the crankshaft. If pendulums attached to a crankshaft are configured so that their natural frequency is equal to a resonant frequency of a vibration mode of the crankshaft, whenever the crankshaft undergoes a vibratory excitation of that frequency and would otherwise begin to resonate, the attached, or coupled, pendulums begin to also resonate at the same frequency and will absorb, not merely damp by friction, the resonant vibrations of the crankshaft. In other words, the resonant vibrations now take place by more harmless interactions between the crankshaft and pendulums, and not by harmful movements of the crankshaft against its bearings and housing. The non-jet engine prior art has extended this use of centrifugal pendulums by recognizing that the centrifugal force field created by a rotating machine component can be used to configure the pendulums so that their resonant frequency changes linearly with engine speed. If done properly, the resonant frequency of the pendulums will always equal the stepwise increasing resonant frequency of a particular vibration mode of the crankshaft whenever the engine speed reaches a point where an aspect of engine design causes a regularly occurring vibratory excitation, i.e., a speed line, to then equal a resonant frequency of the crankshaft. Centrifugal pendulums, therefore, can track a speed line and perform as a so-called dynamic vibration absorber to protect against resonant vibration at all engine speeds. This is a substantial advantage over other vibration attenuation approaches which may protect at only a specific engine speed or for a single vibration mode.
A particular problem in the non-jet engine prior art of centrifugal pendulum absorbers is designing pendulums that will exhibit useful resonant vibration frequencies while not also being too large or too heavy to be practical. Prior art solutions typically involve finding ways to make pendulums of sufficient mass small enough, and not ways to avoid having to use pendulums of large mass.
As previously stated, the jet engine prior art has tried attaching at least pendulum-like structures to blade tips to attenuate vibration. An example of this approach is described in U.S. Pat. No. 4,460,314 to Fuller for a vibration damper. The Fuller patent describes a closed U-shaped chamber inside the tip of a turbine blade. The chamber is partially filled with a liquid that can oscillate from one end of the chamber to the other. The Fuller patent describes equations for determining or selecting the dimensions and other parameters of the damper so that, at a particular frequency and a particular rotation speed, blade vibration will be damped. It also includes a teaching of how those parameters may be varied to produce less damping, but over a greater range of engine speeds. The Fuller patent does not include a teaching of how it might be modified so that it could track a speed line to fully damp blade vibration over all engine speeds.
The Fuller patent describes how its derived equations theoretically indicate that the range of liquid masses and chamber sizes possible in an engine blade can produce practically useful levels of damping. Unfortunately, the severe space and weight limitations for jet engine components are a substantial barrier to successful means for attenuating blade vibration. The problem is even greater for dynamic vibration absorption.
Thus it is seen that there is a need for a successful method and apparatus for attenuating vibration in jet engine blades over all engine speeds, and in particular, a need for a successful adaptation of the teachings of centrifugal pendulums to jet engine blades and other turbomachine components.
It is, therefore, a principal object of the present invention to provide a successful centrifugal pendulum vibration absorber for jet engine blades that works within the space, mass and environmental limitations of a jet engine or other turbomachine.
It is a feature of the present invention that its use of a distributed pendulum instead of a pendulum having its axis of motion at an end allows use of a smaller and lighter pendulum making practical the use of a centrifugal pendulum for a jet engine blade.
It is another feature of the present invention that its damped embodiment is very tolerant to mistuning.
It is an advantage of the present invention that it will absorb vibrations from a hot jet engine blade as well as from a cool jet engine blade.
These and other objects, features and advantages of the present invention will become apparent as the description of certain representative embodiments proceeds.